In a modern, energy-optimized pulp mill, there is a surplus of energy. With today's process, bark can be exported while the remaining energy surplus, in the form of mixtures comprising other burnable residues, is burned in the recovery boiler, with a relatively low efficiency with regard to electricity production. There is also often a problem that the heat transfer capacity in the recovery boiler is a narrow sector, which limits the production of pulp in the mill. The recovery boiler is the most expensive (instrument) unit in the pulp mill.
Separation of lignin from black liquor is an interesting solution to these problems. In this way, the energy surplus can be withdrawn from the process in the form of a solid biofuel and can be exported to e.g. a power station, where the fuel can be used more efficiently than in the recovery boiler of the pulp mill. This lignin is also a valuable material for production of “green chemicals”. Further, lignin extraction leaves a black liquor for combustion with a lower thermal value, which in turn leads to a lower load on the recovery boiler. This gives in a short term perspective possibilities for increased pulp production. In the long perspective lower instrument cost for the recovery boiler is expected.
There are several possible procedures for such a separation, and industrial applications have been known for a long time. Already in 1944, Tomlinson and Tomlinson Jr were granted a patent (U.S. Pat. No. 664,811) for improvements to such a method. The separation method used today is to acidify the black liquor so that the lignin is precipitated in the form of a salt. The solid phase is separated from the liquor and can thereafter be cleaned or modified. There are industrial applications in operation today where lignin is separated from black liquor for use as special chemicals. One example of such a process is the precipitation of lignin from black liquor by acidification with carbon dioxide. The suspension is taken to a storage vessel for conditioning of the precipitate after which the solid lignin is separated and washed (with acidic wash water) on a band filter, and is finally processed to the desired state.
However, the present methods make use of high amounts of acidifying chemicals for separating lignin, which in turn may be used for fuel. Such procedures are thus very expensive and it would accordingly be of great benefit if it would be possible to reduce the amount of acidifying chemicals necessary for separating lignin. Accordingly, there is a need for a method where lignin can be separated using small amounts of acidifying chemicals, e.g. sulphuric acid or carbon dioxide.